The pharmaceutical industry requires the measurement of particle size distribution for pharmaceutical production. The United States Pharmacopoeia (USP) sets mandatory limits on the size and number of particles allowed per volume of liquid. USP standards <788> Particulate Matter in Injections and <789> Particulate Matter in Ophthalmic Solutions require particulate tests and set limits for particulate content in, respectively, injections and ophthalmic solutions. The USP also sets out methods and procedures for conducting these tests.
One instrument that performs this testing is the HIAC Model 9703 Liquid Particle Counting System, shown in FIG. 1, available from Hach Ultra Analytics, Inc., of Grants Pass, Oreg., the assignee of this patent application.
A user of the Model 9703 fills a sample beaker or bottle from sample stock, degasses the sample, and then mixes the sample using (a) a magnetic stirring device built into the instrument; (b) a glass stir bar, or (c) hand agitation/swirling/mixing. The user then positions the open beaker on the instrument, and positions an intake arm into the sample within the beaker. The instrument then automatically withdraws sample fluid and tests it for particulate contamination. It uses the principle of light extinction (light blockage or obscuration) to detect particles in the range of 1.3 μm to 400 μm. The Model 9703 can use a variety of sensors in order to be able to size particles through this range. For submicron particle counting applications, the user may use the MicroCount series sensors, also from Hach Ultra Analytics.
Samples of pharmaceuticals to be tested for particulate contamination are stirred before sample testing. Samples, including blanks or known samples used for instrument validation, also should be stirred during sample testing. The USP does not dictate the exact method of stirring, although both manual and automated means are acceptable. The stirring should be such that the particle counting reproducibly sizes and counts particles in the material under investigation.
One stirring method uses a glass-stirring rod inserted into the sample, which is contained in an open beaker. Another typical stirring method inserts a magnetic stir rod, which may be Teflon coated, into the liquid sample, which is contained in an open beaker. The magnetic bar is inserted into the liquid and sinks to the bottom of the beaker. Under a plate that supports the beaker is a second magnet rotated by a motor, which turns the submerged stirring rod by magnetic coupling, thereby stirring the liquid sample. The Model 9703 comes equipped with a magnetic stirring motor. Another mixing method is to swirl the bottle by hand, without using a stirring rod.
One effect of this practice is that the material in the stirring rod itself may react with the sample under test.
Another effect is cross-contamination from transferring the magnetic or other stirring rod from one sample to another. The stir rod may be encased in a polymer “pill” to prevent metallic contamination from the rod itself, but the polymer is porous and can entrap particles or other material and transfer them from one sample to the next, despite cleaning.
Another effect is that material from the environment may become lodged on or trapped in the stirring rod. Thus, inserting the rod in the sample may transfer particles or other material from the environment to the sample to be tested.
Another effect is that the beaker or other container containing the sample is open to the atmosphere. The mixing process can increase the rate of atmospheric contamination by disturbing the liquid surface and drawing settled surface contamination into the liquid.
The stirring motion can cause other effects. A pure circular motion, like that resulting from a magnetic stirring rod, creates a symmetrical velocity profile. FIG. 2 is a still photograph of a spinning sample in a standard sample bottle 210, having a mouth 220, and containing a mixing rod 230. The photograph is taken from directly over the top of the bottle and looking down through approximately 3 inches (7.6 centimeters) of water. The bottle is filled with water 240 contaminated with 300 μm polystyrene spheres 250 and illuminated with a radial light source (not shown). The stirring rod 230, sits on the bottom of the bottle. The brightly lit area is reflection from the mouth 220 of the bottle. The current practice is to insert a sample tube into the liquid from the top of the bottle and draw the sample from the center of the liquid into the particle counter. FIG. 2 demonstrates the circular nature of this method and the particle void 260 created in the center of the liquid. This particle void 260 is created by a combination of centrifugal forces acting on the polystyrene spheres 250, of the circular liquid flow lines and of the symmetric velocity profile.
Because the bottle is stationary and the liquid is rotating, the velocity at the bottle wall is nearly zero, as is the velocity at the bottle center. FIG. 3 depicts this velocity profile 320 induced by a magnetic stirrer or spinning magnetic rod 330. The hydrodynamic drag is proportional to the square of the velocity. The velocity gradients 340 will compel particles to collect about the bottle radius/2 flow lines, where the fluid velocity is a maximum 350. Additionally, centrifugal forces (not shown) acting on the particles will compel them toward the outside of the bottle. The balance of these counteracting forces determines the radial distribution of particles around the central axis of the bottle.
This stirring motion creates a varying radial particle concentration; a sample drawn from any radial line in the liquid is probably not representative of the particle concentration of the entire sample. Current practice is to draw from the center of the bottle, which is nearly void of particles. This results in underestimating the level of sample contamination. Drawing from other parts of the sample may result in overestimating the level of sample contamination. Also, as the liquid is removed without a proportional number of particles, drawing different aliquots from the same sample may result in non-uniform measurements from aliquot to aliquot within the same sample.
Hand stirring or swirling can be non-uniform between different samples or users, and therefore may yield non-reproducible results. If the mixing is too gentle, there may be inadequate mixing in horizontal and/or vertical directions. Overly vigorous stirring may create air bubbles that may cause sample contamination or create optical artifacts. Those artifacts may be incorrectly interpreted in the testing process. For instance, an optical testing process may incorrectly sense those bubbles as particulate contamination.
Hand stirring or swirling may be difficult or impossible to perform while a sample is being withdrawn from the beaker. The inability to mix, or to mix properly, during sample withdrawal may allow the sample to settle, so that portions of the sample that are withdrawn from the beaker may not accurately represent the concentration in the sample of materials to be tested. There may be other effects stemming from variability between users, including different elapsed times between mixing and testing.
Because the intake of the arm resides above the bottom of a flat-bottomed sample container, all the liquid below that level cannot be used for testing, and is wasted. This can be a disadvantage when testing expensive pharmaceuticals or other liquids.